When a film, such as an insulating film to form a sidewall spacer (SWS) of a gate electrode in a transistor, is formed on a substrate of a semiconductor device, there is a need to form the film at a low temperature in order to prevent diffusion of impurities. There is also a need for such an insulating film to have a low dielectric constant in order to provide a small parasitic capacitance and have a high hydrogen fluoride (HF) resistance in order to maintain its shape in a cleaning process after forming the sidewall spacer. A high quality insulating film to meet such a need may be provided by controlling a composition ratio of the film, for example, a percentage of silicon (Si), oxygen (O), carbon (C), nitrogen (N) or the like contained in the film. Moreover, there is an additional need to control conditions for supplying various kinds of process gases for the purpose of controlling the film composition ratio.
A plurality of process gases have been used to form a film such as the above-mentioned sidewall spacer film in order to further reduce a dielectric constant in a low temperature region and improve the HF resistance. However, if for example an amount of gas exceeding an exhaust capacity is exhausted at one time, gas may stay in an exhaust pipe (exhaust line), which may result in deposition of byproducts in the exhaust pipe and increase in a back pressure of a vacuum pump disposed in the exhaust pipe, and hence cause blocking of the exhaust pipe.